Well it depends largely upon what your system can handle. But assuming you can handle every modern game as well as the older ones you clearly play (which is unlikely, but I'll at least present my picks) then I'd suggest these:
Battlefield Bad Company 2 - Realistic War FPS
Mass Effect 1&2 - Sci Fi RPGs
The Elder Scrolls III: Morrowind - Fantasy RPG
The Elder Scrolls IV: Oblivion - Fantasy RPG
Left for Dead 2 - No subscription Online Zombie Shooter
Star Wars KOTOR 1&2 - Sci Fi RPGs
Unreal Tournament GOTY Edition - Sci Fi FPS
World of Warcraft - Monthly subscription based MMO
Rift - Monthly subscription based MMO
Shogun 2: Total War - Real time strategy
Crysis - Sci Fi FPS
Far Cry 1 - FPS
Command and Conquer series - Sci Fi Real time strategy
Just a few suggestions
It would help other posters if you posted whether you just play old games, or whether you have a more powerful rig that can handle the new games as well
-Nih